1. Field of the Invention
This invention relates to a developing apparatus for visualizing an electrostatic latent image formed on an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member.
2. Related Background Art
As the output means of an external apparatus such as a computer, or a copying apparatus, there is an image forming apparatus using the electrophotographic method as shown, for example, in FIG. 6 of the accompanying drawings.
The image forming apparatus has an electrophotographic photosensitive member usually in the form of a drum as an image bearing member, i.e., a photosensitive drum 100, and the surface of this photosensitive drum 100 is uniformly charged by a primary charger 117. Next, a light 123 is applied by an exposure device correspondingly to image information inputted from an external apparatus, and an electrostatic latent image is formed on the surface of the photosensitive drum 100. This latent image is reversely developed by a developing apparatus 140 by the use of a developer T having a frictional charging polarity of the same polarity as the applied voltage of the primary charger 117, and is visualized as a toner image.
This toner image is transferred onto a transfer material P supplied to the photosensitive drum 100 by a transfer charger 114. The transfer material P having the toner image transferred thereto is separated from the photosensitive drum 100, and then is conveyed to a fixating device 126, where the toner image is fixated as a permanent image on the transfer material P. On the other hand, the photosensitive drum 100 has any untransferred developer T remaining on its surface removed by a cleaning device 116 and is used for the next image forming process.
The developing apparatus 140 is a magnetic one-component developing apparatus and a typical example of the construction thereof is shown in FIG. 7 of the accompanying drawings. This developing apparatus 140 contains in a developing container 140A a negatively chargeable magnetic toner containing a magnetic material as the developer T.
The developing apparatus 140 has its inner lower portion compartmented into a first toner agitating chamber A and a second toner agitating chamber B by a partition plate 108, and first and second toner agitating members 109 and 106 are installed in the first and second agitating chambers A and B, respectively. A developing sleeve 102 rotatable in the direction of arrow in FIG. 7 is installed in an opening portion on the first agitating chamber A side of the developing apparatus 140 which faces the photosensitive drum 100 (FIG. 6), and a magnet roller 110 is disposed in this developing sleeve 102. An elastic blade (developing blade) 103 as a toner amount regulating member is fixed to that portion of the developing apparatus which is above the developing sleeve 102, and depends onto and elastically bears against the developing sleeve 102.
The magnetic toner T contained in the developing apparatus 140 is conveyed from the second agitating chamber B to the first agitating chamber A by the rotation of the second agitating member 106 in the direction of arrow in FIG. 7. The first agitating member 109 is rotated in the same direction to thereby rotate the conveyed toner T near the developing sleeve 102 and create the circulation of the toner so that a constant amount of toner may always be supplied to the developing sleeve 102.
The first and second agitating members 109 and 106 each are formed by one of various rotatable plate materials, screws, rollers or the like, and are rotated at an appropriate speed to efficiently convey the toner T. The partition plate 108 present between the first and second agitating members 109 and 106 has its height made proper to thereby make the amount of toner in the first agitating chamber A substantially constant thus contributing to creating the circulation of a constant amount of toner.
If the amount of toner in the first agitating chamber A is too small, the amount of toner circulated near the developing sleeve 102 will also become small and a sufficient amount of toner cannot be supplied to the developing sleeve 102. If the amount of toner in the first agitating chamber A is too great, the toner will excessively fill up and the irregular coat or streaks or the like of the toner will become liable to occur on the developing sleeve 102. Accordingly, the height of the partition plate 108 is made proper so as to provide a medium amount. A great amount of toner T is contained in the second agitating chamber B.
The developing sleeve 102 comprises a cylindrical member of a non-magnetic electrically conductive metal such as aluminum or stainless steel, and in the present example, it has a diameter of 16 mm.
The developing sleeve 102 is disposed with a predetermined gap kept between it and the photosensitive drum 100 by a gap regulating member, not shown.
The magnet roller 110 which is magnetic field producing means in the developing sleeve 102 is fixed to the side wall of the developing apparatus 140 and is disposed concentrically and nonrotatably in the developing sleeve 102. The developing sleeve 102 is rotated around the magnet roller 110.
The magnet roller 110, as shown, is provided with four magnetic poles S1, S2, N1 and N2, and the magnetic pole S1 is a developing pole, the magnetic pole N1 is a toner amount regulating pole, the magnetic pole S2 is a toner introducing and conveying pole, and the magnetic pole N2 is a toner blow-out preventing pole.
The magnetic toner T supplied by the first agitating member 109 is carried on the developing sleeve 102 by the magnetic force of the magnet roller 110, and is conveyed toward a developing area opposed to the photosensitive drum 100 with the rotation of the developing sleeve 102. In the course of the conveyance, the toner has its layer thickness regulated by the elastic blade 103 bearing against the developing sleeve 102, and is formed into a thin toner layer 107 prescribed to an amount of toner to be conveyed to the developing area.
The toner T conveyed to the developing area flies and adheres to the electrostatic latent image on the photosensitive drum 100 by a developing electric field by a developing bias applied to between the developing sleeve 102 and the photosensitive drum 100, and visualizes the latent image as a toner image.
Descretely from the magnetic one-component developing apparatus as described above, a non-magnetic one-component developing apparatus has recently been put into practical use as a simple color developing apparatus. This non-magnetic one-component developing apparatus, as shown, for example, in FIG. 8 of the accompanying drawings, can assume a construction resembling that of the magnetic one-component developing apparatus. This non-magnetic one-component developing apparatus 240 contains therein a non-magnetic toner T which is an insulative one-component developer, and in the present example, the non-magnetic toner T is negatively chargeable and contains any one of yellow, magenta, cyan and black pigments.
In a non-magnetic one-component developing method, the supply of the toner T to a developing sleeve 202 by a magnetic force is impossible and therefore, instead of a magnet roller being provided in the developing sleeve 202, a toner supply roller 205 made of urethane sponge which serves also to strip off the toner is installed so as to bear against the developing sleeve 202.
The supply roller 205 bears against the developing sleeve 202 and is rotated in the same direction as the latter to thereby supply the non-magnetic toner T onto the developing sleeve 202 and at the same time, strips off any toner remaining after the development in the developing area off the developing sleeve 202. The toner T supplied to the developing sleeve 202 adheres to and is carried on the developing sleeve 202 by a reflecting force or the like attributable to the frictional charging charges possessed by the toner.
Also, an elastic blade 203 of urethane rubber as a developing blade bears against the developing sleeve 202, and regulates the layer thickness of the toner T carried on the developing sleeve 202, whereby the toner is formed into a thin toner layer 207 of a predetermined amount. The amount of the toner of the thin toner layer 207 formed on the developing sleeve 202 is determined by the contact pressure, contact length, etc. of the elastic blade 203 bearing against the developing sleeve 202.
The elastic blade 203 is a chip blade adhesively secured or welded onto a metallic thin plate 204 of phosphor bronze or stainless steel having a thickness of several 100 .mu.m, and uniformly bearing against the developing sleeve 202 by the elasticity of the metallic thin plate 204. The bearing condition of the developing blade 203 is determined by the material, thickness, angle of approach and set angle of this metallic thin plate 204, and the amount of toner on the developing sleeve 202 after being regulated is of the order of 0.3 to 1.0 mg/cm.sup.2 per surface unit area.
The non-magnetic toner T conveyed to the developing area, as in the aforedescribed magnetic one-component developing method, flies to and develop the latent image on the photosensitive drum 100 under the application of a developing bias, and visualizes the latent image as a toner image.
The toner T will now be described. As methods of manufacturing a toner, there have heretofore been a so-called crushing method of uniformly dispersing a parting agent, a coloring agent, a charge controlling agent or the like comprising resin or a low softening point substance by the use of a pressing kneader, an extruder or a media dispersing machine, thereafter causing it to collide against a target mechanically or under a jet stream to thereby minutely crush it into a desired toner particle diameter, and thereafter sharpening the particle size distribution via the classifying step, and making a toner; a method of using a disc or a multifluid nozzle described in Japanese Patent Publication No. 56-13945, etc. to atomize a molten mixture in the air to thereby obtain a globular toner; a method using a suspension polymerizing method described in Japanese Patent Publication No. 36-10231, Japanese Patent Application Laid-Open Nos. 59-53856 and 59-61842 to directly produce a toner; a dispersion polymerizing method using a water organic solvent capable of dissolving monomers but incapable of dissolving polymers to directly produce a toner; and an emulsion polymerizing method typified by a soap free polymerizing method of directly polymerizing under the presence of a water-soluble polarity polymerization starting agent to thereby produce a toner; and any of these methods can be utilized for the manufacture of the toner.
A toner manufactured by the suspension polymerizing method under normal pressure or under pressure is preferable as the polymeric toner. According to the suspension polymerizing method, fine particle toner of which the particle size distribution is sharp and which has a particle diameter of 4 to 8 .mu.m can be obtained relatively easily and the shape thereof is globular and the surface thereof is smooth.
A description will now be made of coefficients SF-1 and SF-2 showing the shape of the toner. The shape coefficients SF-1 and SF-2 were defined as values obtained from the following expressions by using FE-SEM (S-800) produced by Hitachi Works, Ltd. to sample 100 toner particle images at random, introducing the image information into an image analyzing apparatus (Luzex 3) produced by N1colet Japan Corporation through an interface, and effecting an analysis.
SF-1={(MXLNG).sup.2 /AREA}.times.(.pi./4).times.100 EQU SF-2={(PERI).sup.2 /AREA}.times.(.pi./4).times.100,
where
AREA: toner projection surface, PA1 MXLNG: absolute maximum length, PA1 PERI: peripheral length. PA1 (a) a developer carrying member for carrying a developer thereon and conveying it to a developing area opposed to an image bearing member; PA1 (b) an agitating member from agitating the developer supplied to the developer carrying member; and PA1 (c) stripping means for stripping off the developer having passed the developing area from the developer carrying member;
This SF-1 shows the degree of globular shape, and if it is greater than 140, the toner gradually becomes an indefinite shape from the globular shape. SF-2 shows the degree of unevenness, and if it is greater than 120, the unevenness of the surface of the toner becomes remarkable. If SF-1 exceeds 140 or SF-2 exceeds 120, the fog of the toner may increase or the durability of the toner may be somewhat inferior.
The operational effect of the globular shape of the toner is that the fluidity of the toner is improved and the mechanical stress of the toner is decreased. Also, it becomes possible to obtain transfer efficiency up to nearly 100%. In a toner of an indefinite shape such as a crush toner, if the pressing force is high in the transfer by a transfer roller, the toner is mechanically pressed against the photosensitive drum and the unsatisfactory transfer of the so-called "blanks in characters" becomes liable to occur, but it hardly occurs in the globular toner.
According to the suspension polymerizing method, it is possible to make a toner containing a low softening point substance. Specifically, the polarity of a material in a water medium is set so as to be smaller in the low softening point substance than in a main monomer, and a small amount of resin or monomer which is great in polarity is further added, whereby there can be obtained a toner having the so-called core/shell structure in which the inside low softening point substance is covered with shell resin.
As regards the control of the particle diameter of the toner and the control of the particle size distribution of the toner, a predetermined toner can be obtained by a method of changing the kind and the amount of addition of soft water soluble inorganic salt or a dispersing agent acting as protective colloid, or by controlling mechanical apparatus conditions, e.g. the peripheral speed of a rotary, the frequency of pass, an agitating condition such as the shape of an agitating vane and the shape of a container, or the concentration of a solid component in a water solution.
In a polymeric toner having the core/shell structure, a low softening point substance is used as a core substance, whereby heat fixation by an amount of heat smaller than in the prior art becomes possible. Accordingly, a low softening point substance is preferable as the main component of the core portion, and compounds in which the maximum peak value of the main component measured in conformity to ASTM D3418-8 exhibits 40 to 90.degree. C. are preferable. If the maximum peak value is less than 40.degree. C., the self-cohesive power of the low softening point substance becomes weak and as the result, a high temperature offset property becomes weak, and this is not preferable. If the maximum peak exceeds 90.degree. C., a fixation temperature becomes high, and this is not preferable. Further, when a toner is to be directly obtained by a polymerizing method, granulation and polymerization are effected by water and therefore, if the temperature of the maximum peak value is high, the low softening point substance is deposited chiefly during the granulation and hinders the suspension system, and this is not preferable.
For example, DSC-7 produced by Perkin-Elmer Inc. is used for the measurement of the temperature of the maximum peak value. The fusing points of indium and zinc are used for the temperature correction of an apparatus detecting portion, and the heat of fusion of indium is used for the correction of the quantity of heat. A pan made of aluminum was used as a sample, and for reference, an empty pan was set and measurement was effected at a temperature rise speed of 10.degree. C./min.
Further, by using a substance of a high parting property as a core substance, it is also possible to prevent the fusion of the toner to a fixating roller. Thus, it becomes unnecessary to apply a molding lubricant such as silicone oil to the fixating device and therefore, the construction of the fixating device becomes simple and the fixating device can be made low in cost and maintenance-free.
As the low softening point substance providing the core substance, utilization can be made, specifically, of paraffin wax, polyolefin wax, fischer-tropsch wax, amide wax, high-class fatty acid, ester wax and graft/block compounds of these.
Also, it is preferable to add 2 to 30% by weight of low softening point substance to the toner. If the amount of addition is less than 2% by weight, a burden is required for the removal of the aforementioned remaining monomer, and if the amount of addition exceeds 30% by weight, the union of toner particles is liable to occur during granulation in the manufacture by the polymerizing method, and a toner in which the particle size distribution is wide is liable to be produced, and this is unsuitable.
There is a tendency toward the smaller size of the toner as a part of the recent higher quality of electrophotographic images, but the energy required to crush particles is proportional to the minus square of the diameter of the particles and therefore, it is difficult to make the particle diameter of the toner smaller by the crushing method. In contrast, the polymerizing method produces toner particles by the use of chemical reaction and therefore, according to this method, it is easy to make the particle diameter of the toner smaller and it is also easy to obtain a sharp particle size distribution and thus, the polymerizing method is a toner manufacturing method suitable for the image formation of high quality.
As previously described, according to the one-component developing apparatus, it becomes possible to form a thin toner layer suitable for development on the developing sleeve by a simple construction and therefore, the downsizing and simpler maintenance of the electrophotographic apparatus become possible.
In the above-described one-component developing apparatus, however, image outputting repeated for a long period has led to the problem that image density is reduced or the dot reproducibility of halftone image portions is reduced.
Specifically, in the one-component developing apparatus, in order to form a thin toner layer on the developing sleeve, it is required to let the toner pass the developing blade brought close to or urged against the developing sleeve, but the toner has been changed in its physical property values including chargeability by stress and frictional heat received from the developing blade, and the developing characteristic thereof has been aggravated.
When the developing blade is brought into contact with the developing sleeve, for example, when an elastic blade bears against a rigid sleeve made of a metal or a metallic blade bears against an elastic sleeve, good toner friction chargeability and a uniform thin toner layer are obtained and therefore, an image output of high quality is obtained by a simple construction, but when compared with the toner regulation by a magnetic blade or the like which is not in contact with the developing sleeve, the toner is particularly liable to be deteriorated by the creation of the stress and a great deal of frictional heat when the toner passes the developing blade as described above, and accordingly the compatibility of the higher quality of image and longer life of the developing apparatus is hindered.
The temperature rise by the frictional heat of the toner when it passes the developing blade is said to reach several 10.degree. C. in a microscopic area, i.e., the surface of the toner, and when the toner continuously passes the developing blade while adhering to the developing sleeve, the rise of the surface temperature of the toner becomes still higher and at last, locally melts the surface of the toner and an extraneous additive present on the surface of the toner is embedded into a toner binder to thereby deteriorate the chargeability of the toner or make the cohesive power between toner particles rise. That is, by the toner continuously passing the developing blade, the surface temperature of the toner rises to deteriorate the toner, and a reduction in the density of image and the irregularity of the density of halftone portions are caused.
In the magnetic one-component developing apparatus shown in FIG. 7, the toner on the developing sleeve 102 is present on the developing sleeve 102 unless it is used for the development of the latent image on the photosensitive drum 100 and therefore, it continuously passes the developing blade 103 and thus, the toner is deteriorated. Most ordinary text images are 10% or less in printing rate and in this case, about 90% of the toner on the developing sleeve is exposed to deterioration without being used for development.
Reducing the regulated pressure of the developing blade reduces the quantity of heat produced per passage of the toner and is therefore effective for the prevention of the deterioration of the toner, but reduces also the frictional charge imparting property to the toner and thus, particularly under a high humidity environment, the toner cannot have a sufficient amount of frictional charging charges, and a reduction in the density of image and the scattering of the toner around character images are caused and thus, the compatibility of a higher quality of image and a longer life is difficult.
In the non-magnetic one-component developing apparatus shown in FIG. 8, the charging up of the toner by the toner continuously passing the developing blade 203 is prevented by the supply roller 205 stripping off the toner on the developing sleeve 202. However, the toner thus stripped off is present in the first agitating chamber A and is circulated near the developing sleeve 202 and therefore, as the result, the temperature of the toner has risen to cause the deterioration of the toner.
The use of a globular toner in a one-component developing apparatus leads to the advantages that (1) the fluidity of the toner is improved and (2) the transfer efficiency is improved, and further the use of a globular polymeric toner leads to the additional advantages that (3) it is easy to make the particle diameter of the toner smaller and (4) the particle size distribution of the toner becomes sharp (and accordingly, the distribution of the amount of frictional charging charges of the toner becomes sharp).
Also, if the polymeric toner is given core/shell structure by the suspension polymerizing method, there are added the advantages that (5) the fixativeness and the anti-blocking property of the toner are compatible and (6) by a molding lubricant being used in the core portion of the toner, a member for applying a molding lubricant oil to the fixating roller can be simplified.
However, the globular toner, as compared with a crushed toner which is of an indefinite shape and is very uneven on the surface thereof, is small in friction and liable to slip because it is globular. Therefore, when it is used in the aforedescribed non-magnetic one-component developing apparatus, the globular toner slides away along the surface of the portion of contact between the developing sleeve 202 and the supply roller 205, and continuously passes the developing blade 203 and is deteriorated by the aforedescribed reason.
Also, by increasing the quantity of the low softening point substance in the toner, the quantity of heat necessary in the fixating device can be reduced and the lower power consumption of the electrophotographic apparatus can be achieved, but the threshold value of the temperature at which the surface of the toner is melted and deteriorated also lowers and therefore, the service life of the developing apparatus become shorter.
Also, as shown in Japanese Patent Application Laid-Open No. 58-116559, there is a non-magnetic one-component developing method using not a magnetic toner but a non-magnetic one-component toner. This developing method can provide color images corresponding to the recent demand for color images and further, can use a low-cost and compact developing apparatus and has been widely put into practical use.
FIG. 17 of the accompanying drawings shows an example of a developing apparatus using the non-magnetic one-component developing method.
In this example, the developing apparatus has a developing container 401 containing therein a developer, i.e., a non-magnetic one-component toner, and a developing roller 402 as a developer carrying member rotatably carried on the developing container 401 and rotatively driven in the direction of arrow. A toner supplying and collecting roller 415 as a developer supplying and collecting member and a blade 414 as a developer layer thickness regulating member bear against the developing roller 402. The blade 414 has an elastic member 414b of urethane rubber or the like adhesively secured to that surface of a support member 414a formed of phosphor bronze or the like which is opposed to the developing roller, whereby it has the action of forming a thin layer of toner on the surface of the developing roller 402 and imparting charges to the toner. The toner supplying and collecting roller 415 comprises a mandrel 415a of SUS or the like having its outer peripheral surface covered with an elastic member 415b of urethane foam or the like, whereby it has the action of supplying the toner contained in the developing container 401 to the surface of the developing roller 402 and also, scraping off the toner returned without contributing to the developing step from the surface of the developing roller 402.
By this construction, a thin layer of the non-magnetic one-component toner can be simply and easily on the developing roller 402, and it has become possible to develop a latent image on a latent image bearing member.
However, in the developing apparatus using the above-described non-magnetic one-component developing method, the toner supplying and collecting roller 415 is brought into contact with and rotated and frictionally slides relative to the developing roller 402 to thereby effect the supply and collection of the toner. Also, the imparting of charges to the toner is effected chiefly by the frictional contact when the toner passes the blade 414.
That is, in such a developing apparatus, during the time until the toner in the developing container 401 is used for the action of developing the electrostatic latent image in the developing portion, the mechanical load applied to the toner is very great and the damage thereby is very great as compared with the other developing methods.
Further, depending on the disposed position or the direction of rotation of the toner supplying and collecting roller 415, the toner which has not contributed to development cannot sometimes be collected completely, and in some cases, the toner has remained on the toner supplying and collecting roller 415. The toner remaining on the toner supplying and collecting roller 415 again passes the blade 414 portion and is conveyed to the developing portion, but when such a process is continuously repeated, an extraneous additive or the like controlling the amount of charge and fluidity of the toner is embedded into the interior of the toner by mechanical friction received each time or the accumulation of heat, and this often leads to a deteriorated toner of which the desired charging characteristic and fluidity cannot be obtained.
Such deteriorated toner causes many problems in the image forming process, and for example, when the deteriorated toner contributes to development, bad development by a proper developing characteristic being not obtained occurs or in some cases, bad transfer such as blank areas is caused. Further, the supply of fresh toner onto the developing roller 402 is alienated and a reduction in the amount of applied toner may occur to thereby cause bad density or the like.
Also, if such deteriorated toner fuses to the nip portion of the blade 414 and the surface of the developing roller 402, a bad coat such as streaks will be caused and will hinder the imparting of charges to the toner newly supplied onto the developing roller 402. Thus, an uncharged toner is conveyed to the developing area and bad image formation such as fog or irregularity may occur.
Also, according to the above-described developing apparatus, not only a load applied to. the toner is great, but also a load applied to the developing apparatus itself is great.
Further, when the sponge roller as described above is used as the toner supplying and collecting roller 415, if the frictional contact thereof with the developing roller 402 continues for a long time, the abrasion or damage of the toner supplying and collecting roller 415 itself and the clogging or the like by the toner may occur and thus, the function as the toner supplying and collecting roller 415 may become insufficient and good toner supply may become impossible.
As described above, the contact one-component developing apparatus using the non-magnetic one-component toner can form good toner images by a simple construction, while on the other hand the load applied to the toner and the developing apparatus is great, and as compared with the conventional magnetic one-component developing apparatus and two-component developing apparatus, such contact one-component developing apparatus is remarkably deficient in long-term durability and stability.
Accordingly, the contact one-component developing apparatus is utilized chiefly as a developing apparatus of the cartridge type in which the developing apparatus is bodily interchanged during toner replenishment, and is not so adopted in a developing apparatus of a type which is supplied with a developer as in a copying apparatus.
Also, in recent years, the development of a toner capable of being fixated at a lower temperature has been desired due to a reduction in consumed electric power, and a developing process of low stress corresponding to the toner for low temperature fixation is expected.